1. Field of the Invention
The present invention relates to a fuel cap having a pressure regulating valve that regulates the inner pressure of a fuel tank, and more specifically pertains to a mechanism of improving the flow characteristics of the pressure regulating valve.
2. Description of the Related Art
A known fuel cap includes a cap main body that is screwed to a fuel inlet of a fuel tank, and a pressure regulating valve that is attached to the cap main body to regulate the inner pressure of the fuel tank (see, for example, JP No. 10-278958A). The pressure regulating valve has a positive pressure valve and a negative pressure valve to regulate the inner pressure of the fuel tank within a preset range. The positive valve opens in response to an increase in inner pressure of the fuel tank to be higher than a predetermined level, while the negative valve opens in response to a decrease in inner pressure of the fuel tank to be lower than another predetermined level.
The negative pressure valve has a restriction member on a negative pressure valve plug to increase the differential pressure applied to the valve plug and thereby abruptly heighten the flow rate in the open position of the negative pressure valve. The restriction member, however, undesirably lowers the flow rate in the open position of the positive pressure valve and deteriorates the quick pressure regulation performance of the positive pressure valve.